This invention relates to methods for preparing nondusty water-dispersible granules containing mancozeb. Mancozeb, is presently recognized as the outstanding foliar fungicide for use on crops such as tomatoes but presents problems with dispersion in agricultural sprays. It has frequently been necessary to mix mancozeb with several different surface-active agents in order to obtain proper dispersion and also with stabilizers such as paraformaldehyde and hexamethylenetetramine.
It is customary in preparing mancozeb for use in agricultural sprays to react sodium or ammonium ethylene bisdithiocarbamate and manganese sulfate to form maneb which is then subsequently reacted with aqueous zinc salts. This crude mancozeb product is then prepared for drying by filtration or decantation to remove the sulfate salts such as manganese sulfate and then may be mixed with water. The mancozeb slurry is then pumped to an atomizer leading into a spray drier where water is removed from the mancozeb. Other pneumatic or dispersion driers in which the product in finely dispersed form is dried while being carried in a moving stream of heated gas are also used; for example, a Micro-Drier, Jet-O-Drier and the like.
Various surface-active agents, zinc ions, stabilizers such as paraformaldehyde and hexamethylenetetramine and other additives are then added to the mancozeb product in order to formulate the commercial mixture.
U.S. Pat. No. 3,497,598 incorporated herein by reference discloses an improvement over the art in that the solids concentration of the spray slurry could be increased to about 40% by the preaddition of goulac, a ligninsulfonate. The average particle size of the suspended maneb (or dihydrate) fed to the spray drier is taught to be approximately 15-20 microns.
However, the spray dried agglomerated product obtained is fine (90%&lt;100 microns) and suffers from the disadvantages (including dustiness) during field use per U.S. Pat. No. 3,920,442 incorporated herein by reference; especially Column 2, lines 49-68 through Column 3, line 13.